The present invention relates to a valve for controlling a flow rate of gas, and more particularly to a control valve proper for using in a discharge system and a supply system in a system for manufacturing semiconductors, liquid crystals and others.
In the discharge system of the semiconductor manufacturing system, a needle control valve is used for controlling the flow rate of gas. The needle valve is linearly operated by a diaphragm under the condition of a positioning device, or by a screw rotated by a motor.
In the discharge system of the semiconductor manufacturing system, it is required to control the process pressure of several torrs and cleaning pressure of several hundred torrs. The controllable range of the needle control valve is determined by the control characteristic of the needle and the range-ability (the ratio between a controllable maximum capacity coefficient and a controllable minimum capacity coefficient) of the needle. However, the range-ability of the needle valve is about 15-30 in the linear characteristic and about 50-100 in the equal percentage characteristic when the needle valve is controlled in linear proportion relation. Namely, the control range is narrow, so that a high pressure range can not be controlled by the needle valve.
On the other hand, it is required to shut the passage at a high speed smaller than 2 seconds in an emergency. However, if the needle valve is controlled at such a high speed, hunting occurs. As a result, the pressure fluctuation becomes large, which renders the needle valve impossible to control a minimum range.
The above described problems are not trouble for the needle valve only, other valves, for example the shut off valve, has the same problem.
In addition, since the conventional needle valve can not be used to shut off the passage, a shut off valve must be parallelly used.